Particles of metal compounds have been widely used as functional materials and mainly as functional material for electronics. Particularly, they have been used as phosphor employed for displays such as plasma displays. In order to make their function more effective, for example, in order to make phosphor exhibit high brightness, it is required that particles of phosphor have an almost spherical particle shape and a uniform particle diameter.
JP-A-5-254830, for example, discloses a process for producing spherical fine particles of yttrium oxide having a particle diameter of 0.01 μm to 3 μm by subjecting an aqueous solution of yttrium nitrate and urea to hydrothermal treatment at 135° C. under a pressure of 2 kg/cm2, and subjecting the obtained particles to calcination at 850° C. This process, however, is disadvantageous in high cost for industrial practice because it uses a pressure vessel. Therefore, a process without employing high-pressure conditions is desired.
In addition, JP-A-6-271316 discloses a process for producing particles of phosphor comprising a yttrium oxide-europium oxide complex having an average particle diameter of about 0.140 μm by heating an aqueous solution of yttrium chloride, europium chloride and urea after adjusting its pH to 2.5, preparing particles of a compound containing both of yttrium and europium, and subjecting them to calcination at 850° C. The particles obtained by this process, however, has a small particle diameter and an insufficient brightness as a phosphor. Therefore, a process producing particles of a metal compound having a particle diameter larger than 0.3 μm is desired so that a phosphor having a higher brightness is obtained.
The object of the present invention is to provide a process for producing particles of a metal compound having a spherical particle shape and an average particle diameter larger than 0.3 μm without employing high-pressure conditions.